Hereâ€™s an idea: wall moisture sensor. Water damage can be hard to spot early on and by the time you notice it the damage and rot may be extensive. So add a cheap moisture sensor.
The sensor is a 5cm spike with a nickel-sized head. To install it, you push it through the drywall. It expands a bit inside the wall and this helps fix it in place. Also, the back side of the sensor head is adhesive covered so it sticks to the plasterboard and seals the hole.
Now on to the working bits. It contains a cheap humidity sensor IC, a control chip, and a battery. On the head there is a tiny solar cell, a pair of contacts, and a red LED. The solar cell gets a bit of energy from room lighting and keeps the unit working for more than a decade. When humidity is detected, the LED flashes. Touch the contacts and the LED lights to show it works. The sensor and a control chip are embedded in the spike.
It should be cheap to make and long lasting. Home owners can buy a couple and pop them into walls they want to monitorâ€“walls of finished basements, walls containing plumbing, exterior walls, etc. Installed, the sensor is unobtrusive.
If you think this could be useful and are interested in manufacturing/selling this send an email.
[ ]-plasterboard R [ ] o ||[ ] o ||========* -sensor spike m ||[ ] [ ] Inner wall [ ]